I'm on my second set of battery boxes here (the batteries are still the same), but mine are very tall cells so I had a 'bulging' problem due to insufficient stiffness in the original boxes.
Yours are packaged pretty good the way the are, you only need a box to protect them from stuff (metal !) falling on them and to be able to vent gases to the outside (those gases are explosive). Both of those are safety issues.
The simplest I can think of is a plywood box just a little larger than your batteries, with tbe bottom part reinforced by some 2x4's or so in case you want to move the box as a whole.
The lid should have a vent in it, with a tube that allows gas to go outside of the storage room (and out of the building) to avoid dangerous concentrations of gas during heavy charging.
Temperature is an issue, but it is more of an issue when the batteries are discharged, so keep an eye on charge state when it's cold, with a normal charge/disharge cycle when it's cold around the batteries a simple foam insulation layer on your box (maybe make it double walled) should keep thm well even when it is -30 celsius or so.